By virtue of excellent magnetic properties, Nd—Fe—B permanent magnets find an ever increasing range of application. In the field of rotating machines such as motors and power generators, permanent magnet rotating machines using Nd—Fe—B permanent magnets have recently been developed in response to the demands for weight and size reduction, performance improvement, and energy saving. The permanent magnets within the rotating machine are exposed to elevated temperature due to the heat generation of windings and iron cores and kept susceptible to demagnetization by a diamagnetic field from the windings. There thus exists a need for a Nd—Fe—B sintered magnet having heat resistance, a certain level of coercive force serving as an index of demagnetization resistance, and a maximum remanence serving as an index of magnitude of magnetic force.
An increase in the remanence of Nd—Fe—B sintered magnets can be achieved by increasing the volume factor of Nd2Fe14B compound and improving the crystal orientation. To this end, a number of modifications have been made on the process. For increasing coercive force, there are known different approaches including grain refinement, the use of alloy compositions with greater Nd contents, and the addition of effective elements. The currently most common approach is to use alloy compositions having Dy or Tb substituted for part of Nd. Substituting these elements for Nd in the Nd2Fe14B compound increases both the anisotropic magnetic field and the coercive force of the compound. The substitution with Dy or Tb, on the other hand, reduces the saturation magnetic polarization of the compound. Therefore, as long as the above approach is taken to increase coercive force, a loss of remanence is unavoidable.
In Nd—Fe—B magnets, the coercive force is given by the magnitude of an external magnetic field created by nuclei of reverse magnetic domains at grain boundaries. Formation of nuclei of reverse magnetic domains is largely dictated by the structure of the grain boundary in such a manner that any disorder of grain structure in proximity to the boundary invites a disturbance of magnetic structure, helping formation of reverse magnetic domains. It is generally believed that a magnetic structure extending from the grain boundary to a depth of about 5 nm contributes to an increase of coercive force. See K. D. Durst and H. Kronmuller, “THE COERCIVE FIELD OF SINTERED AND MELT-SPUN NdFeB MAGNETS,” Journal of Magnetism and Magnetic Materials, 68 (1987), 63-75. The inventors discovered in JP-B 5-31807 that when a slight amount of Dy or Tb is concentrated only in proximity to the interface of grains for thereby increasing the anisotropic magnetic field only in proximity to the interface, the coercive force can be increased while suppressing a decline of remanence. Further the inventors established a method of producing a magnet comprising separately preparing a Nd2Fe14B compound composition alloy and a Dy or Tb-rich alloy, mixing and sintering as disclosed in JP-A 5-21218. In this method, the Dy or Tb-rich alloy becomes a liquid phase during the sintering step and is distributed so as to surround the Nd2Fe14B compound. As a result, substitution of Dy or Tb for Nd occurs only in proximity to grain boundaries of the compound, which is effective in increasing coercive force while suppressing a decline of remanence.
The above method, however, suffers from some problems. Since a mixture of two alloy fine powders is sintered at a temperature as high as 1,000 to 1,100° C., Dy or Tb tends to diffuse not only at the interface of Nd2Fe14B crystal grains, but also into the interior thereof. An observation of the structure of an actually produced magnet reveals that Dy or Tb has diffused in a grain boundary surface layer to a depth of about 1 to 2 microns from the interface, and the diffused region accounts for a volume fraction of 60% or above. As the diffusion distance into crystal grains becomes longer, the concentration of Dy or Tb in proximity to the interface becomes lower. Lowering the sintering temperature is effective to minimize the excessive diffusion into crystal grains, but not practically acceptable because low temperatures retard densification by sintering. An alternative approach of sintering a compact at low temperature under a stress applied by a hot press or the like is successful in densification, but entails an extreme drop of productivity.
Another method for increasing coercive force is known in the art which method comprises machining a sintered magnet into a small size, applying Dy or Tb to the magnet surface by sputtering, and heat treating the magnet at a lower temperature than the sintering temperature for causing Dy or Tb to diffuse only at grain boundaries. See K. T. Park, K. Hiraga and M. Sagawa, “Effect of Metal-Coating and Consecutive Heat Treatment on Coercivity of Thin Nd—Fe—B Sintered Magnets,” Proceedings of the Sixteen International Workshop on Rare-Earth Magnets and Their Applications, Sendai, p. 257 (2000), and K. Machida, H. Kawasaki, S. Suzuki, M. Ito and T. Horikawa, “Grain Boundary Tailoring of Nd—Fe—B Sintered Magnets and Their Magnetic Properties,” Proceedings of the 2004 Spring Meeting of the Powder & Powder Metallurgy Society, p. 202. Since Dy or Tb is effectively concentrated at grain boundaries, this method succeeds in increasing the coercive force without substantial sacrifice of remanence. This method is applicable to only magnets of small size or thin gage for the reason that as the magnet has a larger specific surface area, that is, as the magnet is smaller in size, a larger amount of Dy or Tb is available. However, the application of metal coating by sputtering poses the problem of low productivity.
In AC servo motors, for example, a permanent magnet rotating machine with a radial air gap as illustrated in FIG. 1 is used. This permanent magnet rotating machine comprises a rotor 3 including a rotor core 1 and a plurality of permanent magnet segments 2 attached to the surface of the core, and a stator 13 surrounding the rotor 3 to define a gap therebetween and including a stator core 11 having a plurality of slots and coils 12 wound on teeth and received in the slots. In the permanent magnet rotating machine illustrated in FIG. 1, the number of (permanent magnet) poles is six (6), the number of teeth is nine (9), and the arrow associated with a permanent magnet segment indicates a direction of magnetization thereof. With regard to the permanent magnet segments, magnetic orientation is effected in a parallel magnetic field so that a direction of easy magnetization is parallel to the center axis of the magnet segment. The coils are wound on teeth as a concentrated winding and connected in Y connection of three phases: U, V and W phases. The solid circle of a coil denotes that the coil winding direction is forward and the crossing (X) of a coil denotes that the coil winding direction is backward, with respect to the plane of paper.
In AC servo motors and similar motors requiring high precision torque control, the torque must have less ripples. Accordingly, it is undesired that when the permanent magnets rotate, the alignment of stator slots and the permanent magnets causes cogging torque to develop due to variations of the magnetic flux distribution across the gap (i.e., torque without current flowing across the coil) or torque ripples to occur when driven by current flowing across the coil. The torque ripples exacerbate controllability and additionally, cause noise. The cogging torque may be reduced by configuring a permanent magnet segment to a cross-sectional shape that tapers from the center toward transverse ends as shown in FIG. 1. With this configuration, the end portion of a permanent magnet segment which is a magnetic pole switch area developing a noticeable variation of magnetic flux distribution produces a smoothened magnetic flux distribution, reducing the cogging torque.
When electric current flows across coils, magnetic fields are developed in the directions of broad arrows depicted in the stator core region, so that the rotor is rotated counterclockwise. At this point, an aft area of a permanent magnet segment in the rotating direction (a circled area in FIG. 1) is in a situation susceptible to demagnetization because the magnetic field is in an opposite direction to the magnetization of the permanent magnet segment. Demagnetization would not only reduce the driving torque, but also give rise to the problem of increased cogging torque due to locally uneven magnetic field.
End portions of an off-centered permanent magnet are very thin and susceptible to demagnetization. Now the reason why a thin gage magnet is susceptible to demagnetization is described. The magnitude of demagnetization of a permanent magnet is determined by the magnitude of a coercive force and the magnitude of a diamagnetic field at the service temperature. The demagnetization susceptibility increases as the coercive force is lower and as the diamagnetic field is greater. The diamagnetic field is the sum of a self diamagnetic field created by magnetization of a permanent magnet and a reverse magnetic field from the exterior while the self diamagnetic field is greater as the thickness of the permanent magnet in the magnetization direction is reduced.
It was then proposed in JP-A 61-139252 to produce a composite magnet by integrally joining a permanent magnet having a lower coercive force and a higher remanence as a non-demagnetizable portion with another permanent magnet having a higher coercive force and a lower remanence as a demagnetizable portion. This method often leads to a reduced motor output because the permanent magnet having a higher coercive force is inevitably accompanied with a lowering of remanence.
As used herein, the term “off-centered” arrangement means that permanent magnet segments are circumferentially arranged such that a small circle delineating the arcuate portion of the segment is off-centered from a great circle circumscribing the apexes of the arcuate portions of the segments. Reference should be made to a concurrently filed application based on Japanese Patent Application No. 2006-233442.